In medical applications of computer assisted tomography (CAT) it is desirable to achieve scan time of about 0.1 sec in order to image portions of the body during live motion such as heartbeat or respiration. The usual source of X-rays in conventional CAT systems is the Coolidge tube. The entire X-ray tube is translated or orbited or both as necessary to provide X-ray projections for planar image reconstruction. One such design using a conventional X-ray tube mechanically orbited about the patient area on a gantry is set forth in U.S. Pat. No. 3,940,625 by Hounsfield issued Feb. 24, 1976.
The Coolidge X-ray tube has relatively high mass and is not designed to withstand the strong accelerating forces necessary for rapid movement and fast scanning. Use of the conventional X-ray tube limits scan time to a few seconds, too slow for live stop-action images.
An ultrafast CAT scanner has been proposed which is intended to reduce scan time to 0.01 sec. (See "Proposed System for Ultrafast Computed Tomography," by Iinuma, Tateno, Umegaki, and Watanabe, in Journal of Computer Assisted Tomography, Vol. 1, No. 4, 1977, pp. 494-498.) The ultrafast system uses a large bell-shaped X-ray tube containing a circular conic anode surrounding the patient area. A system of deflection coils directs an axial electron beam to the anode and around the anode circle for X-ray generation. The X-rays are directed through the patient area to a circular array of stationary detectors axially displaced from the plane of the X-ray beam. Focusing of the electron beam over the relatively long beam path is difficult and contributes to the complexity and expense of the ultrafast system. Further, the detector array, being out of the X-ray beam plane, gives X-ray projection data for a conic surface and not for a true transaxial plane through the patient area. This produces images of lower resolution than true planar measurements would produce.